The long-term objective is to explore mechanisms underlying the ability to understand speech in noise. The detrimental effects of noise on speech recognition have been documented in several clinical populations including normal-hearing and hearing-impaired persons. Two auditory processes that may be relevant to speech recognition in noise are adaptation of excitation and adaptation of suppression. Both of these types of adaptation have been proposed as mechanisms underlying a phenomenon known as "Enhancement." Enhancement experiments have shown that vowel-like sounds are easier to identify if a complex stimulus is presented first. The specific aims of this research are to systematically explore these two types of adaptation and enhancement using complex signals. Then relationships will be evaluated between these processes and discrimination and identification of speech signals that are presented with competing signals with analogous temporal and intensity conditions. The relationships will be explored in four clinical groups who experience difficulty understanding speech in noise: a) young normal-hearing adults, b) elderly normal-hearing adults, c) young sensorineural hearing-impaired adults, and d) elderly sensorineural hearing-impaired adults. The four proposed phases of the research include evaluation of a) adaptation of excitation, b) adaptation of suppression, c) enhancement with complex stimuli, and d) speech identification and discrimination in continuous and gated competition conditions. Forward-masking experiments will be used in the first three phases to determine changes in threshold as temporal and spectral relationships are varied. Digitally-generated synthetic speech and natural speech stimuli will be presented in the final phase to quantify performance in competition conditions. Fuller understanding of the mechanisms underlying speech reception in noise in these clinical groups may lead to the development of improved rehabilitative techniques such as sophisticated signal processing techniques.